Hypervelocity Nuclear Interceptors for Asteroid Deection or Disruption

2011 IAA Planetary Defense Conference

09-12 May 2011

Bucharest, Romania

Bong Wie

Asteroid Deection Research Center

Iowa State University

Ames, IA 50010, USA

Email: bongwie@iastate.edu

ABSTRACT

An intercept mission with nuclear explosives is the only practical mitigation option against the most probable impact

threat of near-Earth objects (NEOs) with a short warning time (e.g., much less than 10 years). Although a less destruc-

tive, standoff nuclear explosion can be employed in such a last minute intercept mission, the momentum/energy transfer

created by a shallow subsurface nuclear explosion is at least 100 times larger than that of a standoff nuclear explosion.

However, the existing penetrated subsurface nuclear explosion technology limits the impact velocity to less than approx-

imately 300 m/s because higher impact velocities destroy prematurely the detonation fusing devices. Also, a precision

standoff explosion at an optimal height of burst above an irregularly shaped, smaller NEO is a technically challenging

task, especially for intercept velocities of 2 to 30 km/s. Therefore, signicant advances in hypervelocity nuclear inter-

ceptor technology needs to be achieved soon to enable a last minute nuclear deection/disruption mission with intercept

velocities as high as 30 km/s. This white paper briey describes the current as well as planned research activities at the

Iowa State Asteroid Deection Research Center for developing such game changing space technology for mitigating the

most probable impact threat of smaller NEOs with a short warning time.

INTRODUCTION

A growing interest currently exists for developing a national plan to protect the Earth from the future possibility of a

catastrophic impact by a hazardous asteroid or comet. In a recent letter on NEOs from the White House Ofce of Sci-

ence and Technology Policy (OSTP) to the U.S. Senate and Congress, the White House OSTP strongly recommends that

NASA take the lead in conducting research activities for the development of NEO detection, characterization, and deec-

tion technologies [1]. Furthermore, President Obama’s new National Space Policy specically directs NASA to “pursue

capabilities, in cooperation with other departments, agencies, and commercial partners, to detect, track, catalog, and char-

acterize NEOs to reduce the risk of harm to humans from an unexpected impact on our planet.” The Planetary Defense

Task Force of the NASA Advisory Council also recommended that the NASA Ofce of the Chief Technologist (OCT)

begin efforts to investigate asteroid deection techniques. Because of such growing national interests, the NEO threat

detection and mitigation problem has been identied recently as one of NASA’s Space Technology Grand Challenges.

The Asteroid Deection Research Center (ADRC) at Iowa State University has been developing strategies and tech-

nologies for deection or disruption of hazardous NEOs. As the rst university research center in the United States

dedicated to such a complex engineering problem, the ADRC was founded in 2008 to address the engineering challenges

and technology development critical to NEO impact threat mitigation. For a research project funded by NASA’s Iowa

Space Grant Consortium, the ADRC has been developing space technologies for mitigating the NEO impact threats [2].

Although various NEO deection technologies, including nuclear explosions, kinetic-energy impactors (KEIs), and

slow-pull gravity tractors (GTs), have been proposed during the past two decades, there is no consensus on how to reliably

deect or disrupt hazardous NEOs in a timely manner. Furthermore, due to various uncertainties in asteroid detection and

tracking, warning time of an asteroid impact with the Earth can be very short. All of the non-nuclear techniques, including

hypervelocity KEIs and slow-pull GTs, require mission lead times much larger than 10 years, even for a relatively small

NEO. However, for the most probable mission scenarios with a warning time much less than 10 years, the use of higher-

energy nuclear explosives in space will become inevitable. Direct intercept missions with a short warning time will result

in the arrival velocities of 2 to 30 km/s with respect to target asteroids. A rendezvous mission with target asteroids,

requiring an extremely large arrival∆V of 2 to 30 km/s, is totally impractical.

Follower Spacecraft !

with Nuclear Explosives

Leader Spacecraft!

(Kinetic-Energy Impactor)

Shield

Deployable !

Boom (optional)

GN&C!

Sensors

Figure 1: Conceptual illustration of a two-body hypervelocity nuclear interceptor (HNI) system.

Although a less destructive, standoff nuclear explosion can be employed for direct intercept missions, the momen-

tum/energy transfer created by a shallow subsurface nuclear explosion is roughly 100 times larger than that of a standoff

nuclear explosion. However, the existing nuclear subsurface penetrator technology limits the impact velocity to less than

about 300 m/s because higher impact velocities destroy prematurely the detonation fusing devices, although an impact

speed limit of 1.5 km/s has been cited for nuclear earth-penetrator weapons (EPWs). Also, a precision standoff explosion

at an optimal height of burst near a irregularly shaped, smaller NEO, with intercept velocities as high as 30 km/s, is not a

trivial task.

Consequently, a hypervelocity nuclear interceptor (HNI) system concept is proposed in this white paper, which will

enable a last minute, nuclear deection/disruption mission with intercept velocities as high as 30 km/s. The proposed

system employs a two-body space vehicle consisting of a fore body (leader) and an aft body (follower), as illustrated in

Fig. 1. The leader spacecraft provides proper kinetic impact crater conditions for the follower spacecraft carrying nuclear

explosives to make a robust and effective explosion below the surface of a target asteroid body. The surface contact burst

or standoff explosion missions may not require such a two-body vehicle conguration. However, for a precision standoff

explosion at an optimal height of burst, accurate timing of the nuclear explosive detonation will be required during the

terminal phase of hypervelocity intercept missions. Robust nuclear deection/disruption strategies and technologies, to

be employed in a last minute, direct intercept mission, should be further studied, developed, and ight tested/validated.

DEFLECTION VS. DISRUPTION

If an NEO on an Earth-impacting course is detected with a short warning time (e.g., much less than 10 years), the chal-

lenge becomes how to mitigate its threat. For a small asteroid impacting in a sufciently unpopulated region, mitigation

may simply involve evacuation. However, larger asteroids, or asteroids impacting sufciently developed regions may be

subjected to mitigation by either disrupting (i.e., destroying or fragmenting with substantial dispersion), or by altering its

trajectory such that it will avoid impacting the predicted impact location or it will completely miss the Earth. When the

time to impact with the Earth exceeds a decade, the velocity perturbation needed to alter the orbit is relatively small (≈1-2 cm/s). A variety of schemes, including nuclear standoff or surface explosions, kinetic-energy impactors, and slow-pull

gravity tractors, can be employed for such cases [2]. The feasibility of each approach to deect an incoming hazardous

NEO depends on its size, spin rate, composition, the mission lead time, and many other factors. When the time to impact

with Earth is short, the necessary velocity change of a target NEO becomes extremely large.

To date, kinetic-energy impactors or nuclear explosions are the most mature technologies for asteroid deection or

disruption. Both approaches are impulsive and energy rich, in that the nal momentum change can be considerably more

than that present in the original impactor, or in the expanded vaporization layer (from a nuclear explosion). Both methods

are expected to eject some debris, and the amount depends on the material properties of their surface. High porosity affects

the ability to convert the excess energy into additional momentum. Some asteroids like Itokawa have been determined to

have densities (porosity) comparable to terrestrial material with well-characterized shock propagation. Others appear to

Figure 2: A summary of the ideal deection∆V performance characteristics of a standoff nuclear explosion [3].

have very low porosity that may absorb excess energy without the hydrodynamic rebound that can amplify the original

impulse.

Because nuclear energy densities are nearly a million times higher than those possible with chemical bonds, it is

the most mass-efcient means for storing energy with today’s technology. Consequently, even in the standoff mode,

a nuclear explosion is much more effective than all other non-nuclear alternatives, especially for larger NEOs with a

short mission lead time. A summary of the deection ∆V performance characteristics of a nuclear standoff explosion is

provided in Fig. 2 [3]. It is very important to note that any NEO deection/disruption effort must produce an actual orbital

change much larger than predicted orbital perturbation uncertainties from all sources. Also, any NEO deection/disruption

approach must be robust against the unknown material properties of a target NEO.

Another nuclear technique involving the subsurface use of nuclear explosives is in fact more efcient than the standoff

explosion. The nuclear subsurface method, even with shallow burial to a depth of 3-5 m, delivers large energy so that

there is a likelihood of totally disrupting the NEO. A common concern for such a powerful nuclear option is the risk that

the deection mission could result in fragmentation of the NEO, which could substantially increase the damage upon its

Earth impact. In fact, if the NEO breaks into a small number of large fragments but with very small dispersion speeds, the

multiple impacts on Earth might cause far more damage than a single, larger impact.

However, despite the uncertainties inherent to the nuclear disruption approach, disruption can become an effective

strategy if most fragments disperse at speeds in excess of the escape velocity of an asteroid so that a very small fraction

of fragments impacts the Earth. When the warning time is very short, disruption is the only feasible strategy, especially if

all other deection approaches were to fail.

HYPERVELOCITY NUCLEAR INTERCEPTOR (HNI) CONCEPT OVERVIEW

In the mid 1990s, researchers at the Russian Federal Nuclear Center examined a conceptual conguration design of a

rigidly connected, two-segment nuclear penetrator system as illustrated in Fig. 3 [4]. Because this conguration, even

with a fore segment equipped with shaped charge, still limited the impact velocity to less than 1.5 km/s, researchers at

the Central Institute of Physics and Technology in Moscow, Russia also conducted a preliminary simulation study of a

concept for high-speed penetrating subsurface nuclear explosion [5]. The concept employed a fore body followed by an

Figure 3: Conceptual illustration of a two-segment nuclear penetrator system proposed by Russian scientists in 1997,

which still limited the impact velocity to less than 1.5 km/s [4].

aft body (carrying nuclear explosives), allowing an impact velocity of 30 km/s. The fore body impacts the asteroid surface

rst, creating a large crater, followed by the aft body, which penetrates to a depth of three meters. However, a further

simulation of nuclear subsurface explosion and a detailed system-level design were not discussed in [5]. It is now time

to further examine this concept to develop a technically feasible option for mitigating the impact threats of NEOs with a

short warning time.

NASA’s most recent impactor endeavor was the Lunar Crater Observation and Sensing Satellite (LCROSS) mission

designed to investigate the possibility of water on the moon [6]. LCROSS was launched in June 2009, in conjunction with

the Lunar Reconnaissance Orbiter (LRO), as part of the Lunar Precursor Robotic Program. For this mission, LCROSS

did not carry a sophisticated impactor spacecraft; rather it used an SUV-sized Centaur booster rocket from the Atlas V

launch vehicle, as illustrated in Fig. 4. The Centaur booster rocket was successfully red at the Cabeus crater at the lunar

South Pole, with an estimated velocity of 2.78 km/s. The impact was expected to form a crater 20 m in diameter and 4

m deep and excavate more than 350 metric tons of lunar regolith with a plume as high as 10 km from the surface. Four

minutes after the Centaur impact, the LCROSS spacecraft followed the impactor through the dust plume to determine the

composition of the ejected material and relay the information back to Earth.

A preliminary conceptual design of an interplanetary ballistic missile (IPBM) system carrying a nuclear interceptor has

been conducted at the ADRC [7, 8]. The proposed IPBM system consists of a launch vehicle (LV) and an integrated space

vehicle (ISV), as illustrated in Fig. 5. The ISV consists of an orbital transfer vehicle (OTV) and a terminal maneuvering

vehicle (TMV) carrying nuclear payloads. A Delta IV Heavy launch vehicle can be chosen as a baseline LV of a primary

IPBM system for delivering a 1500-kg (≈ 2-Mt yield) nuclear explosive for a rendezvous/intercept mission with a target

NEO. A secondary IPBM system using a Delta II class launch vehicle (or a Taurus II) with a smaller ISV carrying a

500-kg (500-kt yield) nuclear explosive is also described in [7]. An OTV can be used as the fore body KEI spacecraft

when a TMV is the aft body spacecraft carrying nuclear explosives.

The terminal-phase guidance and control of a two-body HNI system, consisting of two separated, but formationying,

spacecraft presents a technically challenging problem, involving high impact velocities, up to 30 km/s, and small, faint

targets [9]. A successful rendezvous mission, for yby or proximity operations, can approach a target asteroid from

any angle. However, precision interceptor missions may require impact-angle control for impact at a specied angle.

Communications with Earth may not be feasible during the terminal phase, so the control scheme must rely on onboard

measurements and computations. The combination of a high velocity and a small target means that an effective guidance

system will require only optical measurements. Recent work shows that the trajectories of fragments from a nuclear

explosion, and the eventual impact locations on the Earth, of the dispersed fragments depend signicantly on the impact

angle of the interceptor. Attaining the desired impact angle while still successfully achieving impact is crucial for a

successful nuclear deection/disruption attempt.

The current study effort at the ADRC will provide requirements and limitations on the intercept mission, including

on maneuvering fuel requirements, interceptor design, relative impact velocity, and GN&C sensors. Every potentially

hazardous NEO will have different orbital characteristics and composition. This research will identify which factors affect

angle-constrained terminal-phase guidance, and what strategies to employ for a variety of scenarios. Most current space

missions do not require a specic encounter angle to be commanded in the terminal phase. Instead, the encounter angle is

Kinetic Impactor (NASA’s Deep Impact Mission)

• Collision with Comet Temple 1 (~5 km) on July 4, 2005

• Mission: dirty snowball or icy dirtball? Not for deflection

demonstration

• 370-kg impactor with 10 km/s relative impact speed; V 0

• Targeting accuracy of 300 m depending on albedo, CoB-CoM,

sun-NEO-impactor phase angle, etc.

Image Courtesy: NASA/JPL-Caltech/UMD Asteroid Deflection

Research Center

LCROSS Mission (2009)

Figure 4: Illustrations of NASA’s Deep Impact mission in 2005 and the LCROSS mission launching the Centaur booster

rocket as a kinetic-energy impactor toward the moon in 2009 [6].

Figure 5: Illustration of a proposed IPBM system architecture [7].

Figure 6: Illustration of the terminal-phase guidance and control problem [9].

chosen as a consequence of the orbit the spacecraft follows from the Earth. Maneuvering thrusters are not typically used

to signicantly change the trajectory of the spacecraft near encounter with an object, but rather to make small adjustments

to make the actual trajectory match the nominal mission trajectory. The current study will evaluate the feasibility of

implementing practical control laws on interceptor spacecraft for real missions, namely high-speed intercept missions

with angle-of-impact constraints enforced in the terminal phase with precision targeting requirements. The fundamental

nature of terminal guidance and control problem is illustrated in Fig. 6. Some preliminary results of developing terminal

guidance and control algorithms for asteroid intercept missions can be found in [9].

It is important to note that the Deep Impact mission has validated the kinetic-impact technology for a relatively large,

5-km target body at an impact speed of 10 km/s in reasonably good lighting condition. Precision targeting of a smaller

(e.g., < 500 m), irregularly shaped target asteroid with an impact speed of 30 km/s in worst-case circumstances needs to

be ight tested/validated/demonstrated in the near future.

An asteroid approximately the size of the asteroid Apophis is considered as a reference target asteroid in the current

study. The model asteroid has a total mass of 2.058E13 kg with a diameter of 270 meters. An ideal nuclear subsurface

explosion of this model was developed by Dr. David Dearborn at Lawrence Livermore National Laboratory. This ideal

model assumed a subsurface explosion in a cylindrical region below the surface of the body by sourcing in energy corre-

sponding to 300 kt [10-12]. It assumed a two-component (inhomogeneous) spherical structure with a high density (2.63

g/cm3) core consistent with granite and a lower density (1.91 g/cm3) mantle. The bulk density of the structures was 1.99

g/cm3, close to that measured for asteroid Itokawa (density = 1.95 g/cm3). The energy source region expands, creating a

shock that propagates through the body resulting in fragmentation and dispersal. The structure of the asteroid was mod-

eled with a linear strength model and a core yield strength of 14.6 MPa. The mass-averaged speed of the fragments after 6

seconds was near 50 m/s with peak near 30 m/s [10-12]. A three-dimensional fragment distribution was constructed from

the hydrodynamics model by rotating the position, speed, and mass of each zone to a randomly assigned azimuth about

the axis of symmetry. While the material representations used have been tested in a terrestrial environment, there are

low-density objects, like Mathilde, where crater evidence suggests a very porous regolith with efcient shock dissipation.

Shock propagation may be less efcient in such porous material, generally reducing the net impulse from a given amount

of energy coupled into the surface. More research in this area is needed to understand the limits of very low porosity.

The preliminary results for the Ap300 model with 15 days before impact [11-16] indicate that only 3% of the initial

mass resulted in impacting the Earth even for such a very short time after interception. The impact mass can be further

reduced to 0.2% if the interception direction is aligned along the inward or outward direction of the orbit, i.e., perpen-

dicular to NEO’s orbital ight direction. Such a sideways push is known to be an optimal when a target NEO is in the

last terminal orbit before the impact. Furthermore, in a real situation, we will probably employ a larger nuclear explosive

device (e.g., 1-2 Mt instead of 300 kt) against a 300-m class target asteroid.

270-m Target

300-kt Subsurface

Intercept-to-Impact = 15 Days Impacting Mass << 1%

Kaplinger, Wie, and Dearborn

AIAA 2010-7982

Figure 7: Preliminary results for high-delity modeling and simulation of orbital dispersion of asteroids disrupted by

nuclear explosives [14].

For a larger 1-km NEO, two basic models (M97 and M20e) were also described in [12]. Both models sourced 900 kt

into a cubic surface region of the same 1 km diameter object, with an initial mass of 1.047 × 109 tons. The difference

is that M97 was a nely zoned model. Approximately 20 seconds after the energy deposition, M97 had 31,984 zones of

asteroid material for 9.6732 × 108 tons, 92.8% of the initial mass. The missing 7.2% was ejected from the mesh at high

speed prior to the end of the hydrodynamics simulation.

Expanding upon these nuclear fragmentation models (Ap300, M97, and M20e) of Dr. David Dearborn, we are cur-

rently developing high-delity nuclear fragmentation models including the effect of hypervelocity impact crater condition

uncertainties, caused by the fore body KEI spacecraft, on the dispersal velocity distribution and the size of each fragment,

to develop optimal intercept/impact strategies for robust nuclear fragmentation and dispersion [17]. The fore body impacts

the asteroid surface rst, creating a large crater, followed by the aft body carrying nuclear explosives, which penetrates to

a depth of several meters. We will rene the “static” nuclear blast models used in [10-16] to assess the overall mission

robustness in employing such impulsive, high-energy nuclear subsurface explosions in the face of various physical mod-

eling uncertainties, especially, caused by the initial kinetic-energy impact crater conditions created by the fore body KEI

spacecraft. Modeling and simulation of this type of complex multi-phase physics problem has never been discussed in the

open literature. The objective of the current study is to validate the overall effectiveness and robustness of the proposed

two-body HNI system.

Some preliminary results of hypervelocity impact modeling and simulation using a GPU (Graphics Processing Unit)

accelerated hydrodynamics code, which is being developed at the ADRC, are shown in Fig. 8 [17]. Further rened results

will then be used for the design consideration of thermal shields of the follower spacecraft.

PARAMETRIC CHARACTERIZATION OF MODELING UNCERTAINTIES

Space missions to deect or disrupt a hazardous NEO will require accurate prediction of its orbital trajectory, both before

and after a deection/disruption event. Understanding the inherent sensitivity of mission success to the uncertainties in

the orbital elements and material properties of a target NEO will lead to a more robust mission design, in addition to

identifying required precision for observation, tracking, and characterization of a target NEO. The unique technical chal-

lenges posed by NEO deection/disruption dictate the level of precision needed in the physical modeling of hazardous

NEOs and the identication of relevant parameters through computational/analytical/experimental studies, remote obser-

vation, and/or characterization missions. Consequently, the uncertainty modeling and its parametric characterization are

of current interest to the planetary defense community. The current study at the ADRC also focuses on the parametric

characterization of various physical modeling uncertainties, especially for nuclear deection/disruption missions.

Figure 8: Preliminary illustrative results for the hypervelocity penetrated subsurface nuclear explosion option [17].

NASA’s NEO Observations Program ofce has been funding NEO characterization studies by Dr. Keith Holsapple at

the University of Washington and Dr. Daniel Scheeres at the University of Colorado at Boulder since 2010. Their studies

are concerned with the fundamental physical understanding and modeling of various physical parameters that inuence

the effectiveness of high-energy impulsive approaches. The effectiveness of impulsive approaches using kinetic impacts or

nuclear explosives is strongly dependent on the mechanical strength and porosity of near-surface regions of the asteroid. In

particular, Dr. Holsapple’s research focuses on a complementary experimental, theoretical and code studies of the effects

on an asteroid of the energy deposition of impacts or explosives. Dr. Scheeres’ research is concerned with the response of

an aggregate, accounting for the complex dynamics of a rubble pile when energy has been added to the system as part of

an impulsive mitigation event.

Because the required degree of physical modeling accuracy strongly depends on the specic mitigation mission types,

the current study emphasizes the parametric characterization of physical modeling uncertainties and their resulting orbital

perturbation effects on the outcome of various nuclear deection/disruption options, such as high- or low-altitude standoff,

surface contact burst, and penetrated subsurface nuclear explosions. The effectiveness and robustness of each option in

the presence of signicant physical modeling uncertainties needs to be further examined.

Space missions requiring nuclear deection/disruption of NEOs are in general concerned with: i) robust predictability

of the sufcient miss distance for a successfully deected NEO; ii) robust predictability of the fragments impacting on

the surface of the Earth (for a worst-case situation with a very short warning time); iii) a reliable assessment of reduced

impact damages due to a last minute disruption mission; and iv) an accurate modeling of ∆!V (magnitude and direction

of velocity change) within desired error bounds. Uncertainties in mass, density, porosity, material strength, and other

physical parameters can substantially inuence the outcome of any nuclear deection/disruption attempt. Therefore, a

detailed study is needed to characterize these uncertain parameters, especially for robust nuclear deection/disruption

mission design.

Also, we need to characterize, computationally and/or analytically, the modeling uncertainties and the resulting orbit

perturbation effects in terms of effective∆!V uncertainties and/or uncertain perturbations in orbital elements (∆a,∆e,∆i,∆Ω,∆ω,∆M0) as well as dispersion velocities of fragments. In particular, the uncertainty associated with initial dispersal

velocity and mass distribution of fragments needs to be rigorously modeled and characterized for robust disruption mission

design. Self gravity of fragments also needs to be included in the high-delity dispersion modeling and simulation.

Finally, we need to increase communication and interaction among NEO deection research engineers and NEO

characterization research scientists through building a consensus on the necessary reliable models in the face of signicant

physical modeling uncertainties as well as the practical mission constraints.

CONCLUSION

A concept of using a fore body (a leader spacecraft) to provide proper kinetic-energy impact crater conditions for an aft

body (a follower spacecraft) carrying nuclear explosives has been proposed in this paper as a technically feasible option

for the most probable impact threat of NEOs with a short warning time (e.g., much less than 10 years). The current

as well as planned studies at the ADRC would enable an important step forward for this area of emerging international

interest, by nding the most cost effective, reliable, versatile, and technically feasible solution to the NEO impact threat

mitigation problem, which is now one of NASA’s Space Technology Grand Challenges.

ACKNOWLEDGMENT

This research has been supported by a research grant from the Iowa Space Grant Consortium and in part by the Vance

Coffman Endowed Fund. The author would like to thank Dr. David Dearborn at Lawrence Livermore National Laboratory

for his technical advice and support in the area of nuclear fragmentation modeling and simulation.

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